Mismatched zones

[lostdirection]

Bit of advice really, although I suspect theres not a great deal I can do about it.

We have 2 zones, 1 that covers most the house radiators the other zones covers the big radiator in the loft conversion.
The 2nd zone I think was needed for building control.
Its a new oil boiler.
The issue is that when the 2nd zone fires up the boiler, its return feed is very quickly back to the boiler which switches it off due to the temperature.
The radiator though is mostly stone cold. This cycling continues.
The main zone works fine

Short of joining the 2 zones together can I do any tricks?

 

[Scott_D]

Is the loft rad full of air?
Trv and lockshield valve open?

 

[Chuck]

Short of joining the 2 zones together can I do any tricks?

Check that the automatic bypass valve has been correctly set, from you description I suspect it hasn't. Otherwise...

A minor modification to the wiring so that the zone 2 valve doesn't call for heat but closes when the loft conversion is warm enough might be what you are looking for.

You may be able to improve matters if the thermostats have a 'multizone' mode that synchronises the operation of the zone valves.

The 'proper' solution to this problem is a buffer tank and/or a boiler with a lower minimum output power. The former is a bit pricey and needs space. The latter is just expensive.

Unfortunately, complying with UK Building Regs doesn't always result in an optimal system from a performance point of view.

 

[lostdirection]

Is the loft rad full of air?
Trv and lockshield valve open?

fully bled and its on locksheilds which are fully open, im thinking of closing up the return a bit to reduce the return flow

 

[lostdirection]

Check that the automatic bypass valve has been correctly set, from you description I suspect it hasn't. Otherwise...

A minor modification to the wiring so that the zone 2 valve doesn't call for heat but closes when the loft conversion is warm enough might be what you are looking for.

You may be able to improve matters if the thermostats have a 'multizone' mode that synchronises the operation of the zone valves.

The 'proper' solution to this problem is a buffer tank and/or a boiler with a lower minimum output power. The former is a bit pricey and needs space. The latter is just expensive.

Unfortunately, complying with UK Building Regs doesn't always result in an optimal system from a performance point of view.

Thanks for that, I did think about a buffer tank, annoyingly a binned a stainless tank id had lying around for sometime about a months or so ago!
Bypass is just barely set, so its in the minimum it can be.
Its just too small of a loop and the boiler is only about 8 meters away from it id guess.
I can lower the boiler output with a different nozzle, but then I guess I may have issues with the working zone?

 

[Chuck]

Bypass is just barely set, so its in the minimum it can be.

I assume this means it's closed as much as possible and only passes when the zones have just closed? If so, that's okay. If it's passing most of the time then that's not okay.

Anyway, try tweaking the wiring of zone two so it is controlled by the thermostat but does not call for heat. It's easy to try, and easy to reverse, and works remarkably well unless you really wish to heat only zone 2 for long periods when zone 1 is off.

 

[John.g]

Does the loft rad heat up if (assuming you can) you run both zones together?.

 

[lostdirection]

Does the loft rad heat up if (assuming you can) you run both zones together?.

It does, yes

 

[John.g]

Note the setting on that ABV then shut it fully (clockwise) and fire up the boiler on zone 2 and see if it heats up now. Can you also sometime post a photo of your circ pump/make/model.

If you have a sealed system ensure cold pressure 1.3/15 bar, if open vent ensure feed and expansion tank "full".

 

[lostdirection]

Note the setting on that ABV then shut it fully (clockwise) and fire up the boiler on zone 2 and see if it heats up now. Can you also sometime post a photo of your circ pump/make/model.

If you have a sealed system ensure cold pressure 1.3/15 bar, if open vent ensure feed and expansion tank "full".

Hi John

There is no heat on the output of the bypass valve, its not open

Vented system tank is filled appropriately

 

[John.g]

With zone 2 only open is there any water pitching over through the vent or any sign of movement in the tank.
what is the pump make/model and what mode/setting is it at.

Also with pump off is there water coming from the rad vent and does water still come out the vent with pump on.

I have a rad in a attic converted bedroom (open vented system but the F&E tank is mounted above the CWST) and it always runs with a negative pressure (vent draws in air when running) but it certainly heats up OK, but the 20kw oil boiler does cycle rapidly.

 

[lostdirection]

With zone 2 only open is there any water pitching over through the vent or any sign of movement in the tank.
what is the pump make/model and what mode/setting is it at.

Also with pump off is there water coming from the rad vent and does water still come out the vent with pump on.

I have a rad in a attic converted bedroom (open vented system but the F&E tank is mounted above the CWST) and it always runs with a negative pressure (vent draws in air when running) but it certainly heats up OK, but the 20kw oil boiler does cycle rapidly.

Hi John

I wonder if we are looking too much into this, seems to me fairly straight forward in that when zone 2 is activated the return feed exiting the radiator within minutes is very hot, this then gets back to the boiler which turns off due to the high temperature..it then cycles this repeatedly.

 

[John.g]

You said the rad is virtually cold.
IMO, the rad should still get quite/very hot if there is adequate flow through it, the water just doesn't by pass the rad along the bottom, it rises up very close to the inlet, along the top and flows down to the return. You also spoke about a buffer, well you have one with your boiler, all oil fired boiler heat exchangers hold around 20/25 litres which act as a buffer, the boiler stat (mounted in the top of this "buffer") will have a hysteresis of ~ 10C so the rad contents + the buffer make OF boilers much better at cycling (which they must do always) than gas fired which only have a buffer of a couple of litres. (the Hx). So again IMO, if the boiler flow and return are very hot then the rad should be as well except that there is little or no flow through the rad. Is the zoning done with zone valves or are they separately pumped zones, I would check for any NRV in the system which may be blocked with sludge or stuck, also check that the circ pump is running with zone 2 only on. Its very unlikely but also check that zone 2 zone valve isn't in series with another zone valve.
To prove/disprove your own theory you can always just run one rad only on zone 1 and see if gets/stays hot.

 

[Ric2013]

The issue is that when the 2nd zone fires up the boiler, its return feed is very quickly back to the boiler which switches it off due to the temperature.
The radiator though is mostly stone cold. This cycling continues.

Not sure what 'mostly stone cold' means and can you elaborate please?

If the boiler fires and sends hot water out, it should reach the radiator, and from what I can gather, in your case, it does. The return from the radiator should be from the bottom such that it is the coolest water is that returned to the boiler (heated water will tend to rise to the top). You say, however, that the return feed is very hot. It's not something stupid like your flow goes into the bottom of the radiator and the return comes from the top is it?

Please confirm or deny that the return, in your case, comes from the bottom of the radiator.

Can you please describe whether your radiator is plumbed with valves at top and bottom opposite ends, valves at top and bottom same end, or valves both at the bottom at opposite ends.

 

[lostdirection]

Please confirm or deny that the return, in your case, comes from the bottom of the radiator.

Can you please describe whether your radiator is plumbed with valves at top and bottom opposite ends, valves at top and bottom same end, or valves both at the bottom at opposite ends.

Both valves are at the bottom, opposite ends of the radiator, I've never known them anywhere else

 

[John.g]

That's correct, on zone 2 only and boiler cycling are the flow and return at the boiler very hot? and is the return measured say 2m from the boiler very hot? also is the flow at the rad very hot and the return very cold/cool?

 

[lostdirection]

That's correct, on zone 2 only and boiler cycling are the flow and return at the boiler very hot? and is the return measured say 2m from the boiler very hot? also is the flow at the rad very hot and the return very cold/cool?

Hot everywhere, only cool bit is the top 80% of the radiator

 

[John.g]

Does this mean that the rad is only heating the bottom 20% (all along). If so, check the rad vent, if no water then identify the rad return and keep throttling this until you get air then water from the vent. (think you suggested this earlier?) The probable problem is that the pump setting for this one rad is now too high coupled with the very small head from the F&E tank to the rad. That is why my F&E tank is as high as possible I think.

 

[John.g]

That's correct, on zone 2 only and boiler cycling are the flow and return at the boiler very hot? and is the return measured say 2m from the boiler very hot? also is the flow at the rad very hot and the return very cold/cool?

Did a few late night checks on my attic rad, I have ran it for the past 30 years with lock/shield valve (on return) full open (TRV on inlet), last night I opened the TRV fully and cracked open one rad vent, no water as expected, I then just shut the L/shield fully until water issued, I reckon that this rad runs ~ 80% full all the time (no problem with heating though) I then just kept inching the L/shield open until water venting almost stopped, it is ~ 1/4T open now, returned TRV to its normal setting and will monitor its performance.
I also have a A rated circ pump running in proportional pressure (PP) mode which slows the pump down as TRVs throttle in or zones shut off, if you have one of these you might check its mode/setting.

 

[Ric2013]

Both valves are at the bottom, opposite ends of the radiator, I've never known them anywhere else

You haven't lived . Joking aside, at least we've excluded reversed flows and returns. It has been known.

80% of the top of the radiator being cold, with both the flow to, and return from, the radiator being hot does indeed sound like air air issue, but very odd that, (if) once filled with air, it then reverts to working normally when both zones are running.

They haven't fitted an autobleed valve somewhere (perhaps even in the offending radiator itself) and the pump is causing a vacuum and dragging in air? Shouldn't normally happen with modern systems, but, as you still have an open-vent, I'm assuming you still have some legacy plumbing.

At this point, I think John. is on to something with his talk of pumps. It would be interesting to know where your pump is in relation to the vent, the expansion pipe, and the boiler, and whether you have a pumped flow or pumped return.

 

[John.g]

TBOE was the original (and best?) and is still used in some installations, until somebody discovered that BOE works just as well, it still amazes me that when you admit hot water to a (BOE) rad how it immediately rises to the top and then flows downwards, very like the TBOE.

I don't know if air builds up in a tight rad, judging by mine, it may do eventually but anytime I open my attic vent rad there is a very slight vacuum in there so perhaps has to be some air there? but the rad runs perfectly satisfactorily for months/years on end in this manner. I do have get rid of all the air initially and then it runs away fine.
I just checked it now with the L/shield just cracked open to keep the rad full and it is practically cold, as soon as reopened the L/shield fully, rad back to normal.

 

[Ric2013]

Hi John,
I must admit you lost me in previous threads. Is your point that your loft rad only fills by virtue of pump pressure as it is lower than your F&E cistern and you suspect the OP has the same problem?

 

[John.g]

The top of my rad is ~ 0.5M above the attic floor and the water level in the F&E tank is ~ 1.5M above the floor level so no problem in filling the rad, its only with the circ pump on that the "vacuum" appears, a circ pump head (pressure) is the differential head between the suction and discharge, a pump with a 3M head could have a suction head of -1M and a discharge head of 2M with a open system due to pipe losses and any rad high up (like in the attic) can have negative pressure with the circ pump on.
The F&E tank is normally sitting on the attic floor with its level only slightly higher than the rad top but the rad should still fill with the pump off, don't know the OPs set up with the pump off.

 

[Ric2013]

I don't get why a 2-zone system should be a problem as such: I designed a system for my mother's house that had 5 heating zones, each controlling a single radiator and it didn't have this problem.